BRASILIA, Brazil — Just when you thought it was safe to stock the aquarium . . . here comes a terrifying new breed of mutant creature — half goldfish, half piranha!

“It’s a swimming pit bull,” warns Dr. Raimundo Tubarao, chief marine biologist for the Brazilian Department of Marine Studies. “It looks like a domestic goldfish, except it’s three times as large and it has all the savage instincts and razor-sharp teeth of a piranha. It makes the Siamese fighting fish look like a garden slug.”

Dr. Tubarao says the new species — dubbed “Goldjaws” — was developed at the Marine Studies laboratories by researchers who impregnated goldfish eggs with piranha sperm in an effort to create goldfish that would live longer and fight harder instead of dying in three weeks and having to be flushed down the toilet.

“They succeeded beyond their wildest expectations. Not only did they spawn a vicious hybrid, they created one that reproduces at three times the rate of a guppy.”

A female guppy can reproduce every 26 days, with a brood size that varies from 50 to 100, but “Goldjaws” pops out up to 300 tiny killers every two weeks, says Dr. Tubarao.

“They reach maturation within a month and can grow up to a foot long. Their appetite is tremendous. They relish raw meat, other fish — and each other if no other food source is available.”

They also relish human flesh, as more than a dozen now-maimed goldfish owners and breeders found out after buying what were marketed as “super fish” from pet stores.

The stores had unknowingly caught the fearsome finned killing machines in local ponds and rivers, where they had been dumped by the frightened researchers.

“Those killer goldfish cleaned out my aquarium and took off my right index finger and a chunk of my palm when I tried to scoop them out,” says fish hobbyist Maria Dedos. “I’d bought them thinking they were a larger, more exotic form of goldfish. The next thing I knew, I was minus a finger and $8,000 worth of rare, exotic fish, not to mention the thousands of dollars in aquarium pumps and filters that they also chewed through.”

Fears that “Goldjaws” may spread beyond Brazil’s borders have caused a temporary embargo of any Brazilian marine exports, but Dr. Tubarao says it may already be too late.

“We’ve had reports from Argentina and inquiries from as far away as Florida — apparently some people are interested in breeding them as attack fish. But we’re doing our best to locate and exterminate every one of them.”